Probably the easiest and most reliable fix is to just replace it. Surely it's not that expensive? Or do you just need a solution now?

I'd try common liquid Cyanoacrylate (super-glue), mixed with bicarbonate of soda. Yes, sounds bizarre, but it is remarkably effective at making repairs on semi-rigid plastics. Liquid glue works better than gel types.

The tank must be super clean to ensure a decent bond, although the same's true for pretty much any adhesive. After cleaning with a solvent of your choice, coat the area you want to repair with a thick layer of super-glue. Sprinkle with bicarb, another layer of glue, another layer of bicarb, building up the layers until the hole is filled.

That is certainly THE fix but it doesn't leak. Filled it about 3,000 miles ago and that is the level now.

What if the crack gets bigger, hmm?

Duct tape would be the po' man's fix. The kind that has the stronger adhesive. But they're cheap enough so I'd just replace it outright and be done with it. Pretty much same take as on keener's radiator leak issue.

It should be made from high density polyethylene. The only thing that reliably sticks to polyethylene is more polyethylene ( worked in plastic fabrication for a few years). Hot glue as a filler and a hot butter knife to smear it around will solve the problem. I have patched bigger holes on tanks with that system.